The electromagnetic wave absorbers are grouped by various aspects such as, for example, principles, structures or configurations and respectively have advantages in the operation properties such as a responsive frequency range or the amount of absorption, good weather durability or easy for fabrication. The electromagnetic wave absorbers are generally evaluated in both of the electromagnetic wave absorbing properties and the frequency band range responsive thereto. In detail, when an electromagnetic wave 1 is obliquely radiated to the electromagnetic wave absorber 2 laminated on a metal plate 3 at angle a1 with respect to the perpendicular plane 4, the electromagnetic wave 1 is reflected from the electromagnetic wave absorber 2 at angle a2 with respect to the perpendicular plane 4, thereby forming the reflection 5. The electromagnetic wave absorbing properties are defined by measuring the amount of decay between the incident electromagnetic wave 1 and the reflection 5. If the angle a1 is equal to zero, the electromagnetic wave absorbing property is called the perpendicular incident property, however, others are called as the oblique incident properties. If the angle a1 is increased in value, the electromagnetic wave absorbing properties are deviated from those at zero. In practical applications, the electromagnetic waves are radiated thereto at various angles, then the oblique incident properties are more important than the perpendicular incident property for the electromagnetic wave absorber. Moreover, since the electromagnetic waves are radiated thereto at various frequencies, it is preferable for practical applications that the electromagnetic wave absorber be operative with all of the frequencies. However, the prior-art electromagnetic wave absorbers are limited to a relatively narrow range. Electromagnetic wave absorbers are sometimes classified into the broad bandwidth type and the narrow bandwidth type with the criterion of the specific bandwidth of 20%.
If the electromagnetic wave absorbers are grouped by the configurations, they would be largely divided into a sheet-shape group and a pyramid-shape group. The former group, i. e. , the sheet-shape group, is small in thickness and has a flat plane surface, and, for this reason, the electromagnetic wave absorbers of this group are relatively easy for application. The narrow bandwidth type and tend to drastically deteriorate in the oblique incident properties when the incident angle is increased. The electromagnetic wave absorbers of rubber-ferrite system, ferrite-tile system, rubber-carbon system, urethane-carbon system would be classified into the sheet-shape group. The ferrite containing electromagnetic wave absorber is relatively broad in responsive bandwidth. However, it is not enough to use in an electromagnetic wave shielding room because of the insufficient oblique incident properties. In detail, assuming now that a radiation source 6 of electromagnetic waves is placed in an electromagnetic shielding room 7 defined by an electromagnetic wave absorbers 8a, 8b, 8c and 8d as well as a metal floor 8e as shown in FIGS. 2 and 3, the electromagnetic waves 9 are radiated from the source 6 in various directions. Some components 9 of the electromagnetic waves directly proceed toward a receiver 10. However, the other components 11 are reflected from the electromagnetic wave absorber 8. In general, it is preferable in the electromagnetic wave shielding room to allow the components directly proceeding and reflected from the metal floor to arrive at the receiver 10. Then, the other components reflected from the electromagnetic wave absorbers 8a to 8d should be decreased to be as small as possible.
In this situation, the the electromagnetic wave absorber 8c is expected to be superior in the perpendicular incident absorbing property, however, it is desirable for the other electromagnetic wave absorbers 8a and 8d to be superior in the oblique incident absorbing properties. As to the electromagnetic absorber 8b, the components fall in not only the perpendicular direction but also various oblique directions, and, for this reason, the electromagnetic wave absorber 8b is expected to be superior in all of the electromagnetic wave absorbing properties. However, the electromagnetic wave absorbers 8a and 8d are designed to be similar in absorbing properties to the electromagnetic wave absorbers 8c, because no electromagnetic wave absorber of the sheet-shape type is sufficient in the oblique incident properties. This results in deterioration in electromagnetic wave shielding characteristics such as the site-attenuation properties. The perpendicular incident absorbing property is deteriorated by decreasing the electromagnetic wave in frequency, and, accordingly, the oblique incident properties are also deteriorated with the frequency.
On the other hand, the later group or the pyramid-shape group is of the broad bandwidth type due to the complicated surface thereof, and, for this reason, the electromagnetic wave absorbers of this group effectively absorb the electromagnetic waves radiated at various oblique incident angles. However, since the pyramid protrusions should be at least a quarter of the wavelength in length, the electromagnetic wave absorbers are liable to be large in size and, accordingly, inconvenient in usage. For example, when the pyramid-shape electromagnetic wave absorber is applied to building an electromagnetic wave shielded room, the pyramid-shaped electromagnetic wave absorber reduces the size of shielding room.